Update dependency cisco/libsrtp to v2.4.0 (#41)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [cisco/libsrtp](https://github.com/cisco/libsrtp) | minor | `v2.3.0` -> `v2.4.0` |

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### Release Notes

<details>
<summary>cisco/libsrtp</summary>

### [`v2.4.0`](https://github.com/cisco/libsrtp/compare/v2.3.0...v2.4.0)

[Compare Source](https://github.com/cisco/libsrtp/compare/v2.3.0...v2.4.0)

</details>

---

### Configuration

📅 **Schedule**: At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box.

---

This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).

Reviewed-on: #41
Co-authored-by: renovate-bot <bot@walbeck.it>
Co-committed-by: renovate-bot <bot@walbeck.it>

Dockerfile

@@ -3,7 +3,7 @@ FROM debian:buster-slim@sha256:c8152821b158dd171b4acf92afb0a58fc2faa179a7e0af8ac
 # renovate: datasource=github-tags depName=meetecho/janus-gateway versioning=semver
 ENV JANUS_VERSION v0.11.3
 # renovate: datasource=github-tags depName=cisco/libsrtp versioning=semver
-ENV LIBSRTP_VERSION v2.3.0
+ENV LIBSRTP_VERSION v2.4.0
 # renovate: datasource=git-tags depName=https://gitlab.freedesktop.org/libnice/libnice versioning=semver
 ENV LIBNICE_VERSION 0.1.18
 # renovate: datasource=git-tags depName=https://libwebsockets.org/repo/libwebsockets versioning=semver